When a deposit is put into production or is already in production, it may be advantageous to provide complete isolation (or separation) between different production zones existing along a well. There are various different reasons why this may be so, and in particular:
(a) local regulations require it;
(b) the fluids (or effluents) produced by the various zones are incompatible or at pressures which are incompatible;
(c) the pressures of the fluids in the different production zones are changing at different rates; and
(d) water or gas is being mixed with the fluid produced by at least one of the production zones.
Production zones are isolated by sealing off the space existing between a portion of the rock or inside wall of a well and a corresponding portion of casing inside the well. (Although the present specification uses the term "casing" throughout, it will be understood that the invention is equally applicable to providing sealing between the rock wall of a well and production "tubing".)
In the prior art, sealing is obtained by cementing the above-specified space, sometimes making use of auxiliary sealing means referred to as inflatable cementing packers.
These packers are constituted by annular sealing rings comprising a double elastomer wall reinforced with a metal braid. The double wall delimits a chamber which can be inflated by cement (or oil or water) to occupy up to 1.6 times the nominal diameter of said walls, with a maximum allowed pressure being about 110 bars to 120 bars, as determined by a system of safety valves.
However, the cementing around casing is often not completely leakproof, and ensuring proper sealing can be a long and uncertain operation. Although inflatable cementing packers facilitate obtaining proper sealing by cementing, they nevertheless suffer from limitations and drawbacks, in particular with respect to:
(i) inflation which cannot exceed the value specified above;
(ii) a reduction in the allowable pressure at full diameter, which pressure may be only about 70 bars;
(iii) difficulties in adapting to the outline of the production well which must not include changes in curvature which are too sudden; even when a well has a suitable outline sealing is not always sufficient because the relatively stiff wall of the packer does not completely fill irregularities in the rock surface (with the packer being stiff because of its reinforcing metal braid);
(iv) in wells of conventional diameter with casing of conventional nominal diameter, regions may occur locally where the thickness of the space between the well and the casing is smaller than the radial thickness of the packer when at rest, thereby running the risk of ramming and damage to the packer as it is lowered down the well around the casing; and
(v) the packer is inflated only after cementing has taken place, i.e. after the casing has been centered (or otherwise) and it may not be possible for the packer to properly seal the space around an excentric casing.
The object of the present invention is therefore to provide a method of making a sealing ring in situ between a portion of the inside wall of a well and a portion of the outside wall of casing inside the well, said method meeting practical requirements better than previously known methods for the same purpose, in particular in that:
(A) the casing is properly centered relative to the well;
(B) the sealing ring obtained by performing the method completely fills the annular space between the two selected corresponding portions of well wall and well casing;
(C) the sealing ring effectively withstands pressure variations due to varying operating conditions (e.g. due to stimulation, fracturing, etc. . . .);
(D) the sealing ring withstands chemical attack from the well effluent; and
(E) where necessary, it is easy to restore leakproof sealing.